The present invention relates generally to an improved implantable electrode for delivering electrical stimulation pulses to an organ, and more particularly to an improved implantable electrode means for delivery of electrical stimulation pulses to the heart.
Heart pacer therapy and technology has expanded intensively, and with improvements available in batteries and circuitry, a need has now arisen for improvement of the functional characteristics of the electrode. Recent improvements in batteries and a corresponding reduction of current consumption of the circuitry within the pulse generator have made it both desirable and necessary to improve electrode performance, particularly improvements relating to the current requirements of the electrode. Furthermore, most of the complications resulting from heart pacer therapy may be traced to the electrodes, and thus physical properties appear to also require improvement.
Studies have been conducted, and certain improvements in electrodes and leads have been made in the past. Specifically, metal fatigue has presented problems which may result in lead fractures. Furthermore, improved lead sealing and electrode geometries have been developed, with one typical lead structure being disclosed in co-pending application Ser. No. 635,909, filed Nov. 28, 1975, now U.S. Pat. No. 4,033,355, and entitled "Electrode Lead Assembly for Implantable Devices", assigned to the same assignee.
Porous electrode structures have been known in the past, with the pores being formed in a solid structure and having a size of less than approximately 100 microns. Such structures have been of assistance, particularly in the compatibility of the porous structure to accommodate ingrowth into the channels of active heart tissue. Further improvements are available from the electrode structure of the present invention, with these improvements including both electrical performance and accommodation of desirable heart tissue ingrowth. In electrical performance, improvements are found particularly in the reduction of polarization. Stimulation thresholds have been significantly reduced, with those of the present structure having been found to be as low as one third to one half of the requirements of conventional electrodes. The electrode of the present invention consists of a filamentary member, wherein the individual filaments may be compressed together to form a substantially solid body, and wherein the filaments preferably have a diameter of less than about 100 microns and form less than approximately 20% of the entire volume of the filamentary or fibrous electrode. In one embodiment, the individual filaments may be compressed of compacted within a grid enclosure, with the individual elements forming the grid being slightly larger than the filaments forming the confined filamentary electrode, with the grid enclosure being adapted to confine, restrict, or otherwise retain the filamentary electrode intact.
Organ stimulation, particularly heart stimulation, may be described in terms of electrical field theory. Excitation is initiated by virtue of an electrical stimulus which exceeds a certain threshold level. The electrical field, therefore, may be characterized as a force being applied to the system. Relating the field strength to the electrode structure, the maximum field strength is believed to be present at the surface of the electrode, with the magnitude of the field being generally inversely proportionally to the electrode size. In ordinary electrodes, the excitable tissue normally becomes spaced from the surface of the electrode by the growth of a barrier or layer of fibrotic material. Accordingly the field strength must be related to that field which is available or effective at the interface which develops between the excitable tissue and the fibrotic tissue. Hence, any reduction in the formation or growth of fibrotic tissue should reasonably be calculated to increase the magnitude of the effective field at the excitable tissue boundary and thus increase the ultimate effectiveness of the electrode.
Furthermore, in normal heart pacers, particularly those of the demand/inhibit type or those which respond in synchrony to a natural heart signal, the electrode performs a sensing function as well as a stimulation function. The sensing impedance of the amplifiers typically used range in the order to 20,000 ohms, and any source impedance will, of course, be in series with the input impedance. The electrode of the present invention permits the source impedance to remain low, thereby enhancing the overall sensing capability of the electrode. As has been indicated, the polarization is exceptionally low, this being believed to be due to the large real surface area available for polarization considerations. However, it has also been found that the effective surface area of the structure is small for stimulation considerations. This appears to provide significant advantages for both operating parameters.